Tomato Late Blight


Late blight is caused by the oomycete Phytophthora infestans. Oomycetes are fungus-like organisms also called water molds, but they are not true fungi. There are many different strains of P. infestans. These are called clonal lineages and designated by a number code (i.e. US-23). Many clonal lineages affect both tomato and potato, but some lineages are specific to one host or the other.

The host range is typically limited to potato and tomato, but hairy nightshade (Solanum physalifolium) is a closely related weed that can readily become infected and may contribute to disease spread. Under ideal conditions, such as a greenhouse, petunia also may become infected.



Phytophthora infestans can overwinter in Minnesota if protected in potato cull piles. Overwintering in a tomato production system is unlikely but infected tomato fruits may give rise to infected volunteer seedlings the following season.

The most common routes of introduction each season are infected potato seed tubers, infected tomato transplants shipped in from other regions, or windblown sporangia (asexual spores) from the south. Under cool, wet conditions, P. infestans can infect and produce thousands of sporangia per lesion in less than five days. These sporangia easily become air-borne, resulting in prolific spread of the pathogen. The disease can potentially destroy entire fields in a short period of time if left unmanaged. Long-distance spread to other fields is also likely, particularly under cloudy conditions.

Management

  • Plant early in the season to escape high disease pressure later in the season
  • Do not allow water to remain on leaves for long periods of time
  • Scout plants often and remove infected plants, infected fruit, volunteers and weeds
  • Sign up to recieve alerts at the USAblight website
  • Plant resistant varieties when possible.
  • Protect the crop with fungicides



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